Machines are known (such as that shown in U.S. Pat. No. 3,893,818 to Mickus) which permit a relatively deep slitting operation by exerting on the two side walls of the disc very high pressures which cause the area which is being slit to be practically plane, but such machines are complex and costly since they call for a considerable power supply and the known art does not ensure a slitting sufficiently deep for subsequently making vehicle wheels.